The machining of crankshafts is a complex operation due to the complex shape of the article and the degree of precision required. In recent years, machines for finishing crank pins by means of an internal milling cutter have been found advantageous for various reasons--see for example Kralowetz et al U.S. Pat. No. 3,880,025 and Berbalk U.S. Pat. No. 3,795,161. In these machines, the pin to be finished is positioned within the opening of an annular milling cutter having teeth on its inner periphery. The cutter is driven in rotation and is fed into milling relationship with the pin, and then driven in an orbital path around the pin to mill the pin surface to the desired final diameter. In this particular operation, by providing the cutter with teeth on its opposed axial side surfaces, the cheeks of the crank pin arms can likewise be milled as the cutter is fed toward and away from the pin.
In most prior art machines, the crankshaft is fixedly positioned relative to the machine frame between a pair of columns and the tool is shifted relative to the machine frame to successively machine or mill the individual pins. This arrangement possesses certain inconveniences in that the cutter and its supporting rotary drive and feeding mechanism, which must be shifted relative to the machine frame in moving from one pin to another, is extremely massive, and thus difficult to precisely position, and the feeding mechanism must be of a relatively complex nature because of the different angular positions of the individual crank pins about the main axis of the crankshaft.
Further, because the cutter must be able to move axially along the stationary crankshaft as it is shifted from pin to pin, the crankshaft normally can be supported only at its opposite ends during the milling operation so that the forces applied by the milling cutter to the crankshaft are frequently applied at a point which is at a substantial distance from the points of support of the crankshaft.
In the machine, which is shown and described in the present application, the milling cutter is mounted for operation in a general vertical plane which is fixed relative to the machine frame and the crankshaft is axially and rotatively indexed relative to the machine frame to successively advance the individual pins to a single machining position relative to the machine frame. With this arrangement, for crankshafts of given finished dimensions, the feeding and orbital motion of the internal milling cutter follows a single fixed path during each successive milling operation, and change-over of the machine, to operate on a new crankshaft of a different throw and/or pin diameter, can be readily accomplished simply by replacing a small number of gage blocks in the feed mechanism. Further, because the milling cutter operates in a fixed vertical plane, a clamping mechanism may be mounted at a fixed location on the machine frame near the plane of operation of the cutter to clampingly grip the main journal portion of the crankshaft which is adjacent to the pin at the machining location so that the crankshaft is rigidly supported during the machining operation by a point of support closely adjacent the location at which the cutting occurs.